Projection device

ABSTRACT

A projection device which includes a plurality of deflectable mirror arrays arranged to spatially modulate incident light. A totally internal reflecting surface is arranged to direct light to and from the deformable mirror array. The device includes on or more dichroic layers arranged at an angle relative to the incident light dependent on the spread of angles on the incident beam relative to the optical axis. Additionally or alternatively, the totally internally reflecting surface may be arranged to direct non-spatially modulated light towards a beam dump.

This invention relates to a projection device. In particular, theinvention relates to a projection device for use in a projection systemin which a projected image is formed by spatially modulating light usingone or more spatial light modulator devices, and then projecting thespatially modulated light on to a display.

A spatial light modulator is an optical component which is controllableto spatially modulate an incident light beam. One class of spatial lightmodulators are active matrix devices, which comprise a matrix ofindividually addressed light modulators each effective to modulate thepart of a light beam corresponding to a pixel of the projected image.

Each light modulator may be a liquid crystal, for example as shown in EP0401912.

Alternatively the active matrix device may comprise an array of thedeflectable mirror devices commonly known as deformable mirror devices(DMDs) as, for example, described in U.S. Pat. Nos. 4,856,863, 4,615,595and 4,596,992. Such deflectable mirror devices comprise an array ofminiature mirrored cantilever beam elements which are electrostaticallydeflectable by electric fields provided across a corresponding array ofelectrodes. The extent of the deflection can be controlled by means ofthe electrostatic potential applied to the electrodes to providevariable degrees of deflection. Alternatively the mirror devices can beoperated in a binary manner by applying predetermined electrostaticpotentials to switch each mirror device between discrete deflectionstates. Grey scale is then displayed by causing each mirror device todeflect to an orientation in which light is directed either towards adisplay screen or towards a beam dump for periods corresponding tochosen multiples of a basic period.

Using an array of such mirror devices, each device being individuallyaddressable, a two dimensional image can be produced. The small size andfast switching times of the mirror devices make them usable at videopicture data rates, enabling the display of television or video movingimages on the display screen.

In a projection system using a deflectable mirror device, the incidentlight beam does not scan, as in an electron beam in a cathode ray tube,but illuminates the whole array of mirror devices at once. Thus aprojection system including one or more deformable mirror devicessuffers the disadvantage that it is necessary to space the light sourceand the projection system from each other such that the light paths oflight from the light source to the deflectable mirror or device, and thespatially modulated light transmitted by the deflectable mirror deviceto the projector system do not cross. This produces limitations on thecompactness and efficiency of the overall system.

EP-A-0364043 and EP-A-0418947 both describe a projection system in whichan assembly of two prisms between which is an air gap defining a totallyinternally reflecting surface is interposed in the light paths to andfrom a reflective liquid crystal spatial light modulator. The totallyinternally reflective surface is effective to transmit one of the lightbeams either passing to or from the spatial light modulator, and toreflect the other of the light beams thus separating the two beams.

Our copending International patent application WO 95/22868 (the contentsof which are incorporated herein by reference) discloses a projectionsystem including a prism assembly including at least one air gapdefining a totally internally reflective surface effective both todeflect incident light onto a deflectable mirror array and to transmitspatially modulated light produced by the deflectable mirror arraytowards a projection lens system. Such an arrangement enables the lightbeams incident on and reflected off the spatial light modulator overlap,thus enabling closer spacing of the light source and projector lens thanwould otherwise be possible.

According to a first aspect of the present invention there is provided aprism assembly for use in a projection system, the prism assemblyincluding at least one totally internally reflective surface effectiveto transmit incident light onto at least one reflective spatial lightmodulator, and to reflect spatially modulated light produced by thespatial light modulator towards a display arrangement.

According to a second aspect of the present invention there is provideda projection device comprising at least one dichroic surface effectiveto split incident light into different colour component beams, and aplurality of reflective spatial light modulators, wherein the means forsplitting the incident light into different colour component beams isincorporated in a prism assembly, said means for splitting light beingarranged at an angle relative to the incident light dependent of thespread of angles of light within the incident light.

The spatial light modulators may be deflectable mirror devices.Alternatively the spatial light modulators may be reflective liquidcrystal devices.

In one particular embodiment in accordance with the invention, there areprovided an equal number of totally internally reflective surfaces tothe number of spatial light modulators.

Alternatively a single totally internally reflective surface may be usedto deflect light onto and deflect light from a plurality of spatiallight modulators.

The totally internally reflective surface may be constituted by an airgap in the prism assembly.

A number of embodiments of the invention will now be described, by wayof example only, with reference to the accompanying drawings in which:

FIG. 1 shows schematically the functioning of a deformable mirror array;

FIG. 2 shows schematically the optical illumination of a mirror withinthe array of FIG. 1;

FIG. 3 is a plan schematic view of a colour optical projection displaysystem incorporating the array of FIGS. 1 and 2;

FIG. 4 illustrates the use of a totally internally reflective surface ina projection device;

FIG. 5 illustrates a modification of the projection device of FIG. 4;

FIG. 6 illustrates schematically a further embodiment of a projectiondevice in accordance with an embodiment of the invention;

FIG. 7 is an explanatory drawing illustrating schematically an incidentbeam on a projection device;

FIG. 8 illustrates a further embodiment of part of a projection devicein accordance with an embodiment of the invention; and

FIG. 9 illustrates a further embodiment of a splitting stageincorporated in a device in accordance with an embodiment of theinvention.

OVERALL OPERATION OF PROJECTION SYSTEM

Referring to FIG. 1, a deformable mirror device array for use in aprojection device in accordance with the invention comprises an array 10of m×n deflectable mirrors M₁₁-M_(mn). Typically, there will be in theorder of 500×500 mirrors for a low resolution display, and 2000×2000mirrors for a high resolution display. The array 10 is connected to anelectrical address circuit 12 which receives a signal from an inputcircuit indicated as 14 to which a colour video signal is input. Theaddress circuit 12 addresses the electrodes (not shown) of each of therespective mirror devices M₁₁-M_(mn), as described in our earlierInternational application number WO92/12506, (the contents of which areincorporated herein by reference).

Each mirror device is pivoted for rotational movement about a pair ofhinges 13 at the diagonal corners of the mirrors as illustratedschematically in relation to mirror M₁₁.

Referring now also to FIG. 2, an incident light beam from a light source16 is directed towards the array 10 at an angle α measured from thenormal to the array of around 20° , the direction of the light beambeing perpendicular to the pairs of hinges 13 of the mirror devices M₁₁to M_(mn). When an individual reflector device M is lying in its restposition, parallel to the plane of the array 10, the incident beam isreflected at a corresponding angle of 20° to the normal along path 24 bin an “off” path leading to a beam dump (not shown in FIG. 2).

When the control signal from the addressing circuit 12 sets the mirrordevice M into a first deflection state at an angle of 10° to the planeof the array 10, the incident beam is reflected out along the normalangle to the array on an “on” path 24 a towards a projection lens anddisplay screen (not shown in FIG. 2 but shown in FIG. 3 as 35 and 34).When the control signal sets the mirror device M into a seconddeflection state at the opposite angle of 10° to the first deflectionstate, the incident beam is reflected out at 40° to the normal along 24c in a second “off” path also leading to the beam dump.

Thus, when viewed along the “on” path 24 a, at any instant, the array 10displays a two dimensional image, those mirrors which have been set tothe first deflection state appearing bright, and those which have beenset to the second deflection state appearing dark.

In order to produce a full colour projection system, the deformablemirror array may be addressed in sequence by light within the blue, redand green wavelength bands, with the mirrors being addressed byappropriate control signals to produce sequential blue, red and greenspatially modulated beams. Integration of each set of three differentlycoloured images projected on the display screen is performed by the eyesof an observers viewing the display screen.

Alternatively, referring now particularly to FIG. 3, a colour projectionsystem may include three separate deformable mirror arrays 30 a, 30 b,30 c each of the general form described in relation to array 10 in FIGS.1 and 2, arranged to spatially modulate light within respectively theblue, red and green wavelength bands in parallel. FIG. 3 illustrates theprinciple of operation of such a colour projection system.

A light source 32, which comprises a high power lamp such as an arclamp, is arranged to generate white light along the incident light pathto the three arrays 30 a, 30 b, 30 c. The array 30 c is arranged todeflect the incident beam such that the “on” path from the array 30 cilluminates a projection screen 34 via the mirror 33 and a projectionlens 35. The incident light path lies in a plane normal to that of thescreen, for example by positioning the light source 32 above the displayscreen 34.

Positioned within the light path of the arrays 30 a, 30 b, 30 c are apair of splitter/combiner mirrors 36, 38 which are at an inclination,rotated about the vertical axis such as to reflect portions of theincident beam to the arrays 30 a, 30 b.

The splitter/combiner 38 nearest to the light source 32 carries adichroic coating designed to reflect the blue light wavelengthcomponents of the incident beam towards the deformable mirror array 30 aand to transmit the remaining red and green light substantiallyunattenuated. The array 30 a is electrically addressed so as tospatially modulate the beam to correspond to the blue wavelengthcomponents of the picture to be displayed. The reflected “on” beam isdeflected in the vertical plane by 20° relative to the beam incident onthe array 30 a, but is substantially horizontally unmodified.

The other splitter/combiner 36 carries a dichroic layer designed toreflect red wavelength components of the incident beam so as to directthe red light to a second deformable mirror device array 30 b, which iselectrically addressed so as to modulate the beam to correspond to thered wavelength components of the picture to be reproduced, the reflected“on” path being deflected 20° in the vertical plane relative to thedirection of the incident beam on the array 30 b.

The remaining green wavelength component light is transmittedsubstantially unattenuated by the splitter/combiner 36, and is reflectedoff the mirror 33 so as to be incident on a third deformable mirrordevice array 30 c. The array 30 c is electrically addressed so as tospatially modulate the beam with the green wavelength components of thepicture to be reproduced, the reflected “on” path again being deflected20° in the vertical plane relative to the direction of the incident beamon the array 30 c.

The spatially modulated green light then passes substantiallyunattenuated back through both splitter/combiners 36, 38 through theprojection lens 35 to be projected onto the screen 34. At the firstsplitter/combiner 36 reached on the return path, the spatially modulatedbeam from the red array 30 b is reflected by the splitter/combiner 36into the same path as the spatially modulated green beam. At the secondsplitter/combiner 38 reached in the return path the spatially modulatedbeam from the blue array 30 a is reflected by the splitter/combiner 38back into the same path as the spatially modulated green and red lightbeams so that the light at the projection lens 35 comprises therecombined spatially modulated light beams. The reflection of thespatially modulated green beam by the mirror 33 causes the beam to havethe same “handedness” as the red and blue spatially modulated beamsproduced by the blue and red arrays 30 a, 30 c which have been reflectedby the splitter/combiners 36, 38.

The arrays 30 a, 30 b, 30 c are positioned such that the optical pathtraversed from each array 30 a-30 c to the screen 34 is the same.

First Embodiment

The systems as so far described suffer the disadvantage that the opticalcomponents must be widely spaced in order to prevent overlapping of theinput and output beams to each spatial light modulator array 30 a,b,c inFIG. 3. This puts a limit on the compactness of the system which can beachieved. This problem can be at least alleviated by the use of a totalinternally reflecting surface, as produced at an interface between twomaterials of different refractive indices, for example at a transitionbetween air and glass. This is used to direct light to and from adeformable mirror array, separating the beams passing to and from themirrors of the array.

An example of a projection device incorporating such a total internallyreflecting surface is shown in FIG. 4. The device is incorporated ineither a monochrome projection system or a system incorporating a colourwheel in which light of different colour components is projectedsequentially onto a single spatial light modulator in the form of adeflectable mirror array 401.

An input beam from a light source (not shown in FIG. 4) is directed tothe deformable mirror array 401 through a prism assembly 403. A mirroredprism face 405 within the prism assembly is effective to direct theincident light onto a totally internally reflecting surface 407 definedby the interface between an air gap 408 within the prism assembly 403and the adjacent prism surface. The totally internally reflectingsurface 407 is effective to reflect the incident light onto thedeflectable mirror array 401 at an angle of 20° to the normal asdescribed in relation to FIGS. 1 and 2.

Spatially modulated light reflected from the deformable mirror array 401along the “on” path 24 a defined in FIG. 2 is directed towards aprojection lens indicated schematically as 409, for projection onto theprojection screen (not shown in FIG. 4). Light along the “off” path 24 bin FIG. 2, is reflected at the corresponding 20 degree angle to thenormal to a beam dump (not shown) in FIG. 4.

It will be seen that the incorporation of the total internallyreflective surface 407 enables separation of the incoming and spatiallymodulated beams in the prism assembly in a shorter distance than wouldotherwise be possible. A larger aperture system is therefore practicablethan would otherwise be possible.

A disadvantage of the device shown in FIG. 4 is that the air gap 408will produce astigmatism in the spatially modulated light passingthrough the air gap 408. FIG. 5 illustrates an embodiment of theinvention in which this problem is overcome. In the embodiment of theinvention shown in FIG. 5, the incoming light from the light source (notshown in FIG. 5) is focused by a condenser lens 500 onto a steeringprism 501 at an angle so as to pass through an air gap 502 definingtotal internal reflective surface 503 within a prism assembly 505. Thislight passes onto a deflectable mirror array 507. Spatially modulatedlight reflected from the deformable mirror array 507 along the “on” path24 a of FIG. 2 is reflected back onto the totally internally reflectivesurface 503 where it is totally internally reflected within the prismassembly onto a mirrored surface 509. The spatially modulated light isthen directed towards a projection lens 511 for projection onto adisplay screen (not shown in FIG. 5).

Light along the “off” path 24 b shown in FIG. 2, is also totallyinternally reflected at the total internal reflection surface 503, butat an angle such that it passes into a beam dump 513 at the edge of theprism assembly 505 either directly or via the totally internalreflective surface 516 at the output surface of the prism assembly.

The beam dump 513 may take several forms. It may take the form of alayer of black glass or other absorber which is bonded or fused to theprism assembly 505. At the surface of the beam dump, remote from theprism assembly 505, there may be attached a copper heat sink or othercooling means (not shown). The heat sink may project out so as to mountonto additional external heat sink components (not shown).

The prism assembly 505 may be made of any convenient optical glass, forexample BK7 optical crown glass. If the beam dump 513 is formed fromblack glass layer 513, this will have a refractive index matched to thatof the prism assembly 505. Thus for example the black glass layer may betype NG1 available from Schott. The thickness of the black glass layer513 will be chosen to be a compromise between the requirement for lightabsorption, and requirement for the heat generated to be conducted awayfrom the prism assembly 505, and will typically be of 0.5 millimetresthickness.

It will be seen that the system illustrated in FIG. 5 has the advantageover the system illustrated in FIG. 4 that the spatially modulated lightalong the “on” path from the deflectable mirror device 507 does not passthrough the air gap 503, but undergoes total internal reflection withinthe prism assembly 505. Thus any astigmatism in the spatially modulatedbeam produced by the air gap 407 illustrated in FIG. 4 is avoided.

It will be appreciated that some of the light directed along the “off”path 24 b will not be directed onto the totally internally reflectivesurface 503 but will be directed towards the output face of the prismassembly 505. This light will be reflected by the output surface 516onto the beam dump 513.

The array 507 may optionally be cemented onto the appropriate facet ofthe prism assembly 505 using a suitable cement, or may be coupled usingoptical coupling fluid. Alternatively the array 507 may be movablerelative to the facets, in order to allow alignment of the array.

The air gaps 503 formed in the prism assembly 505 will typically bearound 15 microns thick. The air gap may be defined by means of recessesin the glass of the prism assembly. Alternatively spacers, for examplemica or loops of fine metal wire, may be used to produce air gaps ofvery accurate spacing. The spacers will be attached to the prismassembly 505 using some form of cement, the choice of which will beobvious to a person skilled in the art of projection systems. Thespacers will be thermally matched to the glass of the prism assembly505.

The light from the lamp 16 may contain substantial power in the infrared and ultra violet frequency bands. The infra red radiation isundesirable because it heats the optical components leading to potentialmisalignment of the optical components. The ultra violet radiation isundesirable as it may affect the cement which holds the prism componentstogether. Thus the front convex surface of the condenser lens 500 whichis used to focus light from the light source onto the prism assembly 505may be coated with a coating 513 which transmits visible light butreflects infra red and/or ultra violet radiation. The surface throughwhich the light beam enters the prism assembly 505 may alsoadvantageously be coated with a filter coating 515 to reduce further thelevel of infra red and/or ultra violet unwanted radiation. Either ofthese coatings may also be designed to trim the spectral distribution ofthe light from the light source which passes into the system.

Second Embodiment

Turning now to FIG. 6, the second embodiment of the invention to bedescribed is incorporated in a colour projection system in which threeseparate deflectable mirror arrays are arranged to spatially modulaterespectively red, blue and green light.

A prism assembly formed of six prisms A, B, C, D, E and F carries threeseparate deflectable mirror arrays 603, 605, 607 each being separatelyaddressable so as to be responsive to spatially modulated incident blue,green and red light respectively. On the prism surface between prisms Cand D, there is formed a first dichroic layer 615 effective to reflectblue light and transmit red and green light. On the prism surfacebetween prisms E and F there is formed a second dichroic layer 617effective to reflect red light and transmit light of other wavelengths.

Between prisms A and B, B and C, and D and E there are formed three airgaps 609, 611 and 613, these defining totally internally reflectivesurfaces 610, 612 and 614 respectively for light incident on thesesurfaces at greater than a critical angle.

It will be appreciated that the arrangement shown in FIG. 6 is in fact athree dimensional prism array, the prisms indicated as A and B being setat an angle of 45° about the axis X, X′ so as to be normal to the hingeaxes of the mirror elements within each of the blue and red DMD arrays603, 607. The schematic form of the illustration in FIG. 6 is for thesake of clarity, the light splitting paths for the red and blue lightbeing omitted.

Incident light from a white light source (not shown) is reflected fromthe totally internally reflective surface 610 between the first twoprisms A and B. The green light wavelength components pass through prismC, dichroic layer 615, prism D, prism E, dichroic layer 617 and prism Fin turn, to be incident on the green deflectable mirror array 605 at theappropriate angle indicated in FIG. 2. Spatially modulated green lightalong the “on” path reflected from the mirror array 605 then passesthrough the prisms F, E, D, C, B and A and through all three air gaps609, 611, 615 to the projection lens (not shown in FIG. 6).

The dichroic layer 615 between prisms C and D reflects the blue lightonto the blue deflectable mirror array 603, whilst the red light isreflected by the dichroic layer 617 between prisms E and F onto the reddeflectable mirror array.

Light from the red deflectable mirror device is totally internallyreflected by the third totally internally reflective surface 614 andthen reflected off the dichroic layer 613 to be recombined in the outputlight path to the projection lens (not shown in FIG. 6). Light from theblue deflectable mirror array 603 is totally internally reflected by thesecond totally internally reflective surface 612 formed between theprisms B and C and reflected by the second dichroic layer 617 to berecombined with the spatially modulated red and green output beams toform a white output beam exiting towards the projection lens (not shownin FIG. 6).

It will be appreciated that as the red and blue spatially modulatedlight undergoes two reflections prior to being recombined to form theoutput white spatially modulated light beam, it is not necessary toprovide a further reflector in the green light path as shown in FIG. 3to produce the same “handedness” for light within each colour channel.It will be appreciated that the prism arrangement shown in FIG. 6provides a particularly compact configuration.

Third Embodiment

It will be appreciated that the light emitted by the light source 16illustrated in FIGS. 1 and 2 will not be a point light source, but willtypically be an arc lamp producing an approximately parallel beam whichis in turn focused by a condensing lens onto the deflectable mirrorarrays. Thus as illustrated in FIG. 7, there will be a range of valuesof the angle β at which the light from the light source 700 is incidenton any of the optical components such as the component 703 up to amaximum value γ which will depend on the design of the input optics,including the path length between the condensing lens 701 and thecomponent 703.

In the case where the component 703 is a dichroic splitting mirrors asillustrated for example as 34 and 36 in FIG. 3, the light may beincident on the dichroic mirrors at an angle which is sufficiently farfrom the normal to the dichroic mirrors that polarization effects causea broadening of the shape of the transmission/reflection spectra, makingsharp precise colour splitting by the dichroic surfaces difficult toachieve, and thus limiting both the efficiency and the colorimetry ofthe system. If the angle of incidence of the light on the dichroicsurface is reduced towards normal incidence so as to reduce thesepolarization effects, the size of the light splitting system willincrease forcing the use of longer, less light efficient, optical paths.As a compromise, an angle of incidence on the dichroic surfaces ofapproximately 30° from the normal may be used, as used, for example inour copending International application WO 95/22868.

Turning now to FIG. 8, this figure is a schematic illustration of acolour splitting system for splitting a multi-component light beamincident on an input prism 801 into light of a particular wavelengthband, which is then directed onto a spatial light modulator indicatedschematically as 803. For clarity the return path from the spatial lightmodulator has been omitted. A dichroic layer 805 is formed in theinterface between two further prisms 807 and 809 forming part of theprism assembly. The dichroic layer 805 is effective to selectivelyreflect light within a particular wavelength band, for example redlight, and reflect the red light towards a totally internally reflectivesurface 811 defined at one surface of an air gap 810 between the inputprism 801 and the second prism 807. The red light is totally internallyreflected from the surface 811 and directed onto the deformable mirrorarray 803. Light within other wavelength bands passes through thedichroic surface 805 through the output prism 809 to, for example afurther splitting system (not shown in FIG. 8).

The inventors have found that if the incident path is analyzed, there isa definite limited range of ray angles that can be used. The dichroicsurface 805 is arranged so as to be set at an angle θ to the normal tothe incident beam which is slightly greater than the greatest angle Ψsubtended by the input beam to the optical axis. A typical value for θis about 7°. The dichroic surface will then be effective to reflect theselected wavelength band light at an angle of 2θ to the optical axis.

Turning now to FIG. 9, in an adaptation of the arrangement illustratedin FIG. 8, a second splitter stage may be incorporated to enableincident white light to be split into red, blue and green colourchannels. A first totally internally reflecting surface 901 is definedat one surface of an air gap 902 between an input prism 903 and a firstoutput prism 905. A second totally internally reflecting surface 907 isdefined at one surface of an air gap 908 between a second output prism909 and the first output prism 905.

A first dichroic layer 911 effective to selectively reflect red lightand to transmit light within all other wavelength bands is formedbetween the second output block 909 and an intermediate block 913. Asecond dichroic layer 915 effective to selectively reflect blue lightand transmit light within all other wavelength bands is formed betweenthe intermediate block 913 and a third output block 917.

Thus in use of the splitter shown in FIG. 9, input white light isdirected into the input block 903, passing straight through the firsttotally internally reflective surface 901. The light is incident on thedichroic layer 908 at a small angle to the normal to the dichroic layer.Red light is reflected from the first dichroic layer 911 to be reflectedback onto the first air gap 901 and totally internally reflected throughthe output prism 905. The red light then passes to the first spatiallight modulator (not shown) which is arranged to spatially modulate redlight.

Light of wavelengths other than red light passes through the firstdichroic layer 911 onto the second dichroic layer 915, also at a smallangle to the normal to the dichroic layer. Blue light within theremaining light is reflected by the second dichroic layer 915 onto thesecond air gap 907 to be totally internally reflected by the surface 907to pass to the second spatial light modulator (not shown) which isaddressed so as to spatially light modulate the blue light.

The remaining green light passes through the second dichroic layer 915,to pass through the output prism 917 to the third spatial lightmodulator (not shown) which is arranged to spatially modulate the greenlight.

It will be seen that due to the particular optical configuration, thegreen light passes directly to the green deflectable mirror device,whilst the red and blue light both undergo a double reflection. Due tothis double reflection compared to the configuration shown in FIG. 3, asin the configuration illustrated in FIG. 6 there is no necessity toprovide a reflective surface for the green light such that the threelight component beams have the same handedness as discussed in relationto FIG. 3.

It will be appreciated that the configurations shown in FIGS. 8 and 9may be adjusted to perform particular configurations dictated by thesystem design. Extra stages can be incorporated in order to eliminateinfrared or ultra-violet light within the input light beam. It will beappreciated however that due to the almost normal incidence of the inputbeam on each of the dichroic surfaces, very small polarization losseswill occur. In particular, it will be appreciated that whilst the bluelight has to undergo a double transition through the first dichroiclayer 911, due to the near normal incidence of the light on the dichroicsurface losses will be minimized. Alternatively, any losses can be usedto fine tune the colorimetry of the system by fine tuning the colourspectrum within the three colour channels. The nested configurationshown in FIG. 9 enables a very compact splitting system to be achievedwhich can be made to be very efficient.

It will also be appreciated that the spatial light modulators may be setat nearly 90° to the input beam. It will also be appreciated that due tothe high efficiency of the system, a further stage may be incorporatedto separate out the third colour component light beam, i.e. green lightin the example shown in FIG. 9, thus allowing unwanted infra-red orultra-violet radiation to pass on and out of the system.

It will be appreciated that in a high power system, the internalreflective surface incorporated in any of the embodiments of theinvention can be produced without an air gap by utilizing thedifferences in refractive index between the two prisms componentsdefining the surface. Thus one of the prism may be sapphire, whilst theother prism may be a silica or other low index glass prism. Whilst thisdoes alter the path alignments of the beams, a stronger assembly isachieved. A further possibility is that in, for example FIG. 9 theoutput prism 905 may be formed of, for example sapphire and emersed in afluid such as silicon oil or water to provide the internal reflectivesurfaces 901, 907. Such an arrangement would be extremely robust inoptical terms and enable the handling of very high power levels.

It will be appreciated that whilst the projection systems shown in FIGS.6, 8 and 9 use the colour splitting scheme shown schematically in FIG.3, other colour splitting schemes may be used in a system in accordancewith the invention, for example a scheme using the three coloursmagenta, cyan and yellow, or using more than three colour componentbeams. A system in accordance with the invention may also use the coloursplitting mirrors to improve the balance of light modulated by thespatial light modulators.

In some applications it may be advantageous to add a further array tothe three arrays used in a colour projection system. Such a furtherarray may be for example used for power handling reasons as for exampledisclosed in the applicant's co-pending PCT application no. WO 95/04582,the contents of which are incorporated herein by reference. Thus, forexample it may be advantageous to arrange for the green colour channelto be split between two arrays.

It will also be appreciated that a single air gap may be used to divertthe unwanted light where an air gap is not used to direct light onto thespatial light modulators.

It will also be appreciated that whilst the spatial light modulatorsarray described by way of example are deflectable mirror devices, aprojection system in accordance with the invention also findsapplication where other reflective spatial light modulators such asreflective liquid crystal devices are used. It will also be appreciatedthat the splitting system illustrated in FIGS. 8 and 9 will also findapplication with a transmissive spatial light modulator, for example aliquid crystal device.

What is claimed is:
 1. A projection device comprising a light source, aplurality of reflective spatial light modulators, and a prism systemarranged in the light path between the light source and the spatiallight modulators, the prism system including at least one totallyinternally reflecting surface for light incident on the surface atgreater than a critical angle, the surface being arranged to directlight from the light source onto a dichroic surface, focusing means inthe light path between the light source and the prism system such thatthe light beam incident on the dichroic surface has a spread of anglesrelative to the optical axis effective to focus the light produced bythe light source to produce a focused beam on each spatial lightmodulator, the dichroic surface being arranged at an angle to the normalto the optical axis substantially equal to said spread of angles, thedichroic surface being arranged to reflect a portion of the lightincident on the dichroic surface onto at least one of the spatial lightmodulators.
 2. A projection device according to claim 1, wherein eachspatial light modulator is a deflectable mirror array, the reflectivesurface of each spatial light modulator is substantially perpendicularto the direction of incidence of light from the light source on theprism system.
 3. A projection device according to claim 1 or claim 2including a plurality of dichroic surfaces effective to split light ofdifferent wavelengths between said plurality of spatial lightmodulators, and to pass unwanted light out of the projection systemtowards a beam dump.
 4. A projection device according to claim 3including two totally internally reflective surfaces, light from thelight source being arranged to pass through the first totally internallyreflective surface to a first dichroic surface between the first andsecond totally internally reflective surfaces; the first dichroicsurface being arranged to reflect light within a first wavelength bandonto the first totally internally reflecting surface for reflection ontoa first of said reflective spatial light modulators, the rest of thelight being transmitted through the second totally internally reflectivesurface to a second dichroic surface in the light path after the secondtotally internally reflective surface; the second dichroic surface beingarranged to reflect light within a second wavelength band onto thesecond totally internally reflective surface for reflection onto asecond of said reflective spatial light modulators and to transmit lightwithin a third wavelength band onto a third of said reflective spatiallight modulators; the second totally internally reflective surface beingarranged to direct spatially modulated light in the return path from thesecond spatial light modulator onto the second dichroic surface forrecombination with spatially modulated light in the return path from thethird spatial light modulator; the first totally internally reflectivesurface being effective to direct spatially modulated light from thefirst spatial light modulator onto the first dichroic surface forrecombination with spatially modulated light in the return path from thesecond and third spatial light modulators to produce a spatiallymodulated output beam.
 5. A projection device according to claim 4,including a third totally internally reflective surface effective todirect light from the light source through the first totally internallyreflective surface, and to direct the spatially modulated output beamtowards a projection screen.
 6. A projection device comprising aplurality of deflectable mirror arrays each comprising an array ofmirror devices each mirror device having a first orientation effectiveto reflect light incident on the mirror device along an “ON” path toform part of a spatially modulated light beam and a second orientationeffective to reflect light incident on the mirror device along an “OFF”path for the array different from the “ON” path, a light source and aprism system for directing light from the light source onto thedeflectable mirror arrays, address means for electrically addressingeach deflectable mirror array so as to cause each mirror device to haveone of the first and second orientations; the prism system including atleast one dichroic surface effective to split light of differentwavelengths from the light source between the deflectable mirror arrays,the prism system further including at least one totally internallyreflective surface for light incident on the surface at greater than acritical angle, the surface being arranged such that light incident onthe surface from the light source is transmitted by the surface to atleast one of the deflectable mirror arrays and spatially modulated lightreflected from the deflectable mirror array along the “ON” path isreflected by the totally internally reflective surface towards adisplay, and light reflected from the deflectable mirror array along the“OFF” path is directed by the totally internally reflective surfacetowards a beam dump.
 7. A projection device according to claim 1 or 6 inwhich at least one of the or each totally internally reflective surfacesis formed at a boundary between two prism members of differentrefractive indices.
 8. A projection device according to claim 1 or 6, inwhich at least one of the or each totally internally reflective surfacescomprises a boundary between a solid material and a liquid.
 9. Aprojection device according to claim 8 in which said liquid is siliconeoil.
 10. A projection device according to claim 9 in which the liquid iswater.
 11. A projection device according to claim 7, arranged such thatspatially modulated light does not traverse said boundary.
 12. Aprojection device according claim 6 including means for removingunwanted wavelengths within the incident light from the spatiallymodulated light.
 13. A projection device according to claim 12 whendependent on claim 1, wherein said removal means comprises a furtherdichroic surface effective to selectively reflect said unwantedwavelengths.
 14. A projection system including a projection deviceaccording to claim 1 and a display screen.
 15. A projection deviceaccording to claim 1 in which the spatial light modulator is a liquidcrystal device.
 16. A projection device according to claim 1 includingmeans for removing unwanted wavelengths within the incident light fromthe spatially modulated beam.
 17. A projection device according to claim16, wherein said removal means comprises a further dichroic surfaceeffective to selectively reflect said unwanted wavelengths.
 18. Aprojection device comprising a light source, a plurality of reflectivespatial light modulators, and a dichroic surface, focusing means in thelight path between the light source and the dichroic surface such thatthe light beam incident on the dichroic surface has a spread of anglesrelative to the optical axis effective to focus the light produced bythe light source to produce a focused beam on each spatial lightmodulator, the dichroic surface being arranged at an angle to the normalto the optical axis substantially equal to said spread of angles, thedichroic surface being arranged to reflect a portion of the lightincident on the dichroic surface onto at least one of the spatial lightmodulators.